St Martin's Le Grand

Green pilot Our attitude to environmentally responsible design has shifted dramatically in recent decades. Similarly, frameworks for assessing environmental impact and sustainability targets have evolved, many of them becoming redundant as our knowledge develops and new technologies become available. The UK’s Net Zero Carbon Buildings Standard, announced in 2024, will bring together a raft of environmental ratings under one umbrella, and has already attracted strong interest from the commercial sector. Our refurbishment of 1 St Martin’s Le Grand has been accepted as a pilot testing scheme for the new standard, setting a benchmark for the retrofit of City buildings.

Untapped potential The building is one of a cluster developed by the General Post Office (GPO) during the nineteenth century. Completed in 1895 by the architect Sir Henry Tanner, it sits just north of St Paul’s Cathedral on an island site next to the famous Postman’s Park. During the 1980s it was gutted by the financial services company Nomura, leaving just the Portland stone façade and steel frame. Despite the brutality of this refurbishment, it has made our current retrofit more straightforward, vastly increasing the possibilities for reconfiguration.

Architecture shaped by history Our architectural approach has been shaped by the site’s history. On a macro level, we needed to work within the viewing corridors to St Paul’s, and the context of Postman’s Park and the many listed buildings in the immediate area. On a micro level, fragments of the walls of Roman Londinium run across the site, and a listed police call box stands at the park entrance. Within these constraints, we have retained the embodied carbon of the existing floors and elevations, and extended them responsibly to give a new headquarters building of around 300,000sqft.

Adding mass We tested many different options for adding additional floors at roof level, informed throughout by conversations with City of London planners. The 1980s roof extension of French-style mansards was inefficient, so our plan is to remove it, setting back the existing floorplates and adding three-storey pavilions to express each corner. Attic storeys are added, each one set back to soften the overall mass. The detailing complements the original architecture, with levels 11 and 12 having a more modernist feel, being almost invisible from street level.

Improved efficiency As found, the interior building was very inefficient. There were 3 cores and a non-level entrance sequence. When trading floors were introduced into the structure in the 1980s they didn’t always match the original window heights, compromising the natural light in places. We have consolidated the cores into a more central one, and repositioned the entrance on St Martin’s Le Grand. This now leads into a double-height reception area that connects visually with Postman’s Park and runs right through the building. Above, the reconfigured cores have created more efficient, flexible floorplates, opening up new views out.

Mapping the façade To grow the new architecture, we had to fully understand Henry Tanner’s original, so we mapped each façade to identify any subsequent alterations. We looked in particular at how he marked the corners of the building, and the symmetry of the main elevations, reapplying the key principles in our new additions. New windows, for example, take the same proportions as the original ones. We also looked to reinstate some of that original symmetry to the St Martin’s Le Grand elevation, placing the main entrance in the centre and introducing a new archway to mirror that at the northern end.